Electric current driving type display device

ABSTRACT

In a pixel circuit  100 , a driving TFT  110 , a switching TFT  115 , and an organic EL element  130  are provided between a power supply wiring line Vp and a common cathode Vcom and a capacitor  120  and a switching TFT  111  are provided between a gate terminal of the driving TFT  110  and a data line Sj. A switching TFT  112  is provided between a connection point B between the capacitor  120  and the switching TFT  111  and the power supply wiring line Vp, a switching TFT  113  is provided between the gate and drain terminals of the driving TFT  110 , and a switching TFT  114  is provided between the gate terminal of the driving TFT  110  and a reference supply wiring line Vs. A potential that brings the driving TFT  110  into a conduction state is applied to the reference supply wiring line Vs. Thus, variations in the threshold voltage of a drive element can be properly compensated for and unwanted light emission from an electro-optical element can be prevented.

TECHNICAL FIELD

The present invention relates to a display device and more particularlyto an electric current driving type display device such as an organic ELdisplay or FED.

BACKGROUND ART

In recent years, there have been demands for thin, lightweight, fastresponse display devices, and accordingly, research and development fororganic EL (Electro Luminescence) displays and FEDs (Field EmissionDisplays) have been actively conducted.

An organic EL element included in an organic EL display emits light withhigher luminance as the voltage to be applied thereto is higher and theamount of current flowing therethrough is larger. However, therelationship between luminance and voltage of the organic EL elementeasily varies due to an influence such as drive time or ambienttemperature. Hence, when a voltage control type drive scheme is adoptedin an organic EL display, it is very difficult to suppress variations inthe luminance of the organic EL element. In contrast to this, theluminance of the organic EL element is substantially proportional tocurrent and this proportional relationship is less susceptible toexternal factors such as ambient temperature. Therefore, it is desirableto adopt an electric current control type drive scheme in the organic ELdisplay.

Meanwhile, a pixel circuit and a drive circuit of a display device arecomposed using TFTs (Thin Film Transistors) made of amorphous silicon,low-temperature polycrystal silicon, CG (Continuous Grain) silicon, orthe like. However, variations easily occur in TFT characteristics (e.g.,threshold voltage and mobility). In view of this, a circuit thatcompensates for variations in TFT characteristics is provided in a pixelcircuit of an organic EL display and by the action of this circuit,variations in the luminance of the organic EL element are suppressed.

Schemes to compensate for variations in TFT characteristics in anelectric current driving type drive scheme are broadly divided into anelectric current program scheme in which the amount of current flowingthrough a driving TFT is controlled by a current signal; and a voltageprogram scheme in which such an amount of current is controlled by avoltage signal. Use of the electric current program scheme enables tocompensate for variations in threshold voltage and mobility and use ofthe voltage program scheme enables to compensate for only variations inthreshold voltage.

However, the electric current program scheme has the following problems:first, since a very small amount of current is handled, it is difficultto design a pixel circuit and a drive circuit; and second, since it issusceptible to parasitic capacitance while a current signal is set, itis difficult to achieve a large-area circuit. On the other hand, in thevoltage program scheme, an influence of parasitic capacitance, etc., islittle and a circuit design is also relatively simple. In addition, theinfluence exerted on the amount of current by variations in mobility issmaller than the influence exerted on the amount of current byvariations in threshold voltage and the variations in mobility can besuppressed to a certain extent in a TFT fabrication process.Accordingly, even a display device adopting the voltage program schemecan obtain satisfactory display quality.

For an organic EL display adopting the electric current driving typedrive scheme, a pixel circuit shown below has been conventionally known.FIG. 17 is a circuit diagram of a pixel circuit described in PatentDocument 1. A pixel circuit 910 shown in FIG. 17 includes a driving TFT911, switching TFTs 912 to 914, capacitors 915 and 916, and an organicEL element 917, All of the TFTs included in the pixel circuit 910 are ofa p-channel type.

In the pixel circuit 910, the driving TFT 911, the switching TFT 914,and the organic EL element 917 are provided in series between a powersupply wiring line Vp (potential is VDD) and a ground. The capacitor 915and the switching TFT 912 are provided in series between a gate terminalof the driving TFT 911 and a data line Sj. The switching TFT 913 isprovided between the gate and drain terminals of the driving TFT 911 andthe capacitor 916 is provided between the gate terminal of the drivingTFT 911 and the power supply wiring line Vp. A gate terminal of theswitching TFT 912 is connected to a scanning line Gi, a gate terminal ofthe switching TFT 913 is connected to an auto-zero line AZi, and a gateterminal of the switching TFT 914 is connected to an illumination lineILi.

FIG. 18 is a timing chart of the pixel circuit 910. Before time t0, thepotentials of the scanning line Gi and the auto-zero line AZi arecontrolled to a high level, the potential of the illumination line ILiis controlled to a low level, and the potential of the data line Sj iscontrolled to a reference potential Vstd. When at time t0 the potentialof the scanning line Gi is changed to a low level, the switching TFT 912is changed to a conduction state. Then, when at time t1 the potential ofthe auto-zero line AZi is changed to a low level, the switching TFT 913is changed to a conduction state. Thus, the gate and drain terminals ofthe driving TFT 911 are equal in potential.

Then, when at time t2 the potential of the illumination line ILi ischanged to a high level, the switching TFT 914 is changed to anon-conduction state. At this time, a current flows into the gateterminal of the driving TFT 911 from the power supply wiring line Vpthrough the driving TFT 911 and the switching TFT 913, and the gateterminal potential of the driving TFT 911 rises while the driving TFT911 is in a conduction state. The driving TFT 911 is changed to anon-conduction state when the gate-source voltage becomes a thresholdvoltage Vth (negative value) (i.e., the gate terminal potential becomes(VDD+Vth)). Therefore, the gate terminal potential of the driving TFT911 rises to (VDD+Vth).

Then, when at time t3 the potential of the auto-zero line AZi is changedto a high level, the switching TFT 913 is changed to a non conductionstate. At this time, a potential difference (VDD+Vth−Vstd) between thegate terminal of the driving TFT 911 and the data line Sj is held in thecapacitor 915.

Then, when at time t4 the potential of the data line Sj is changed fromthe reference potential Vstd to a data potential Vdata, the gateterminal potential of the driving TFT 911 is changed by the same amount(Vdata−Vstd) and thus becomes (VDD+Vth+Vdata−Vstd). Then, when at timet5 the potential of the scanning line Gi is changed to a high level, theswitching TFT 912 is changed to a non-conduction state. At this time, agate-source voltage (Vth+Vdata−Vstd) of the driving TFT 911 is held inthe capacitor 916.

Then, when at time t6 the potential of the illumination line ILi ischanged to a low level, the switching TFT 914 is changed to a conductionstate. Thus, a current flows through the organic EL element 917 from thepower supply wiring line Vp through the driving TFT 911 and theswitching TFT 914. Although the amount of current flowing through thedriving TFT 911 increases or decreases depending on the gate terminalpotential (VDD+Vth+Vdata−Vstd), even when the threshold voltage Vth isdifferent, if the potential difference (Vdata−Vstd) is the same, thenthe amount of current is the same. Therefore, regardless of the value ofthe threshold voltage Vth, a current of an amount according to thepotential Vdata flows through the organic EL element 917 and thus theorganic EL element 917 emits light with a luminance according to thedata potential Vdata.

As such, according to the pixel circuit 910, variations in the thresholdvoltage of the driving TFT 911 can be compensated for and the organic ELelement 917 is allowed to emit light with a desired luminance.

FIG. 19 is a circuit diagram of a pixel circuit described in PatentDocument 2. A pixel circuit 920 shown in FIG. 19 includes a driving TFT921, switching TFTs 922 to 925, a capacitor 926, and an organic ELelement 927. The switching TFTs 923 and 925 are of an n-channel type andother TFTs are of a p-channel type.

In the pixel circuit 920, the driving TFT 921, the switching TFT 925,and the organic EL element 927 are provided in series between a powersupply wiring line Vp and a common cathode Vcom (potentials arerespectively VDD and VSS). The capacitor 926 and the switching TFT 922are provided in series between a gate terminal of the driving TFT 921and a data line Sj. Hereinafter, a connection point between the drivingTFT 921 and the capacitor 926 is referred to as A and a connection pointbetween the capacitor 926 and the switching TFT 922 is referred to as B.The switching TFT 923 is provided between the connection point B and thepower supply wiring line Vp and the switching TFT 924 is providedbetween the connection point A and a drain terminal of the driving TFT921. All gate terminals of the respective switching TFTs 922 to 925 areconnected to a scanning line Gi.

FIG. 20 is a timing chart of the pixel circuit 920. Before time t0, thepotential of the scanning line Gi is controlled to a high level. When attime t0 the potential of the scanning line Gi is changed to a low level,the switching TFTs 922 and 924 are changed to a conduction state and theswitching TFTs 923 and 925 are changed to a non-conduction state. Thus,the connection point B is disconnected from the power supply wiring lineVp and connected to the data line Sj through the switching TFT 922.Also, the gate and drain terminals of the driving TFT 921 obtain thesame potential. Hence, a current flows into the gate terminal of thedriving TFT 921 from the power supply wiring line Vp through the drivingTFT 921 and the switching TFT 924, and the potential at the connectionpoint A rises while the driving TFT 921 is in a conduction state. Thedriving TFT 921 is changed to a non-conduction state when thegate-source voltage becomes a threshold voltage Vth (negative value)(i.e., the potential at the connection point A becomes (VDD+Vth)).Therefore, the potential at the connection point A rises to (VDD+Vth).

Then, when at time t1 the potential of the data line Sj is changed froma data potential Vdata0 for the last time (a data potential written to apixel circuit in an adjacent upper row) to a data potential Vdata forthis time, the potential at the connection point B is changed to Vdata.Accordingly, the voltage between electrodes of the capacitor 926immediately before time t2 is a potential difference (VDD+Vth−Vdata)between the connection point A and the connection point B.

Then, when at time t2 the potential of the scanning line Gi is changedto a high level, the switching TFTs 922 and 924 are changed to anon-conduction state and the switching TFTs 923 and 925 are changed to aconduction state. Thus, the gate terminal of the driving TFT 921 isdisconnected from the drain terminal. Also, the connection point B isdisconnected from the data line Sj and connected to the power supplywiring line Vp through the switching TFT 923. Thus, the potential at theconnection point B is changed from Vdata to VDD and accordingly thepotential at the connection point A is changed by the same amount(VDD−Vdata; hereinafter, referred to as VB) and thus becomes(VDD+Vth+VB).

After time t2, the switching TFT 925 goes into a conduction state andthus a current flows through the organic EL element 927 from the powersupply wiring line Vp through the driving TFT 921 and the switching TFT925. Although the amount of current flowing through the driving TFT 921increases or decreases depending on the gate terminal potential(VDD+Vth+VB), even when the threshold voltage Vth is different, if thepotential difference VB is the same, then the amount of current is thesame. Therefore, regardless of the value of the threshold voltage Vth, acurrent of an amount according to the potential Vdata flows through theorganic EL element 927 and thus the organic EL element 927 emits lightwith a luminance according to the data potential Vdata.

As such, according to the pixel circuit 920, as with the pixel circuit910, variations in the threshold voltage of the driving TFT 921 can becompensated for and the organic EL element 927 is allowed to emit lightwith a desired luminance. In addition, the pixel circuit 920 has anadvantage over the pixel circuit 910 in that the circuit size is smallerdue to the absence of the capacitor 916, the auto-zero line AZi, and theillumination line ILi. Note that in the pixel circuit 920 in order tobring the driving TFT 921 of a p-channel type into a conduction state,the potential difference VB needs to be negative (i.e., Vdata>VDD).

[Patent Document 1] International Publication Pamphlet No. WO 98/48403

[Patent Document 2] Japanese Patent Application Laid-Open No.2005-157308

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, the pixel circuit 920 has a problem that it may not be able toproperly compensate for variations in the threshold voltage of thedriving TFT 921. For example, when almost no current flows through thedriving TFT 921 in a previous frame (when black display is performed),the potential VA at the connection point A at time t0 in FIG. 20 issubstantially (VDD+Vth). When the potential at the connection point B ischanged from VDD to Vdata during the period from time t0 to time t1,accordingly, the potential at the connection point A is also changed.However, since, as described above, Vdata>VDD, if the potential at theconnection point B rises from VDD to Vdata when the potential at theconnection point A is substantially (VDD+Vth), then the potential at theconnection point A becomes higher than (VDD+Vth). Due to this, thedriving TFT 921 is controlled from a state in which almost no current isflown to a state in which further less current is flown, and thus, doesnot go into a conduction state. In this case, variations in thethreshold voltage of the driving TFT 921 cannot be compensated for bythe above-described method.

Patent Document 2 also describes a pixel circuit 930 shown in FIG. 21,in addition to the pixel circuit 920. In the pixel circuit 930, gateterminals of respective switching TFTs 922 and 924 are connected to ascanning line Gi and gate terminals of respective switching TFTs 923 and925 are connected to a control line Ei. According to the pixel circuit930, by changing the switching TFT 924 to a conduction state andthereafter changing the switching TFT 925 to a non-conduction state, thegate terminal potential of a driving TFT 921 can be drawn to a potentialVSS of a common cathode Vcom. At this time, since the driving TFT 921goes into a conduction state, variations in the threshold voltage of thedriving TFT 921 can be compensated for by the above-described method.Note that Patent Document 2 describes the configuration of the pixelcircuit 930 but does not clearly indicate that the pixel circuit 930 isoperated at the above-described timing.

However, if the pixel circuit 930 is operated at the above-describedtiming, when the gate terminal potential of the driving TFT 921 is drawnto the potential VSS of the common cathode Vcom, a current flows throughan organic EL element 927 and thus the organic EL element 927 emitslight. Since the gate terminal potential of the driving TFT 921 at thistime cannot be precisely controlled externally, even if the pixelcircuit 930 is externally controlled, unwanted light emission from theorganic EL element 927 cannot be suppressed. Hence, when the pixelcircuit 930 is operated at the above-described timing, it is difficultto perform precise grayscale display. When black display is performed,too, the organic EL element 927 emits light and thus the contrast of adisplay screen decreases.

In the pixel circuit 920, while the potential of the scanning line Gi isat a low level (during one horizontal scanning period), a process ofcompensating for variations in the threshold voltage of the driving TFTis completed. Therefore, the gate terminal potential of the driving TFT921 (potential at the connection point A) needs to be changed from aprevious potential to a potential (VDD+Vth) for a threshold state duringone horizontal scanning period.

However, the potential VA at the connection point A at time to in FIG.20 varies depending on a data potential written to the pixel circuit 920last time. The potential at the connection point A is, for example,furthest from (VDD+Vth) when the organic EL element 927 emits light atthe maximum luminance before time t0 and closest to (VDD+Vth) when theorganic EL element 927 does not emit light before time t0. However, ineither case, the potential at the connection point A needs to be changedto (VDD+Vth) during one horizontal scanning period. Hence, in a highdefinition display device in which one horizontal scanning period isshort, it is difficult to precisely compensate for variations in thethreshold voltage of a driving TFT.

An object of the present invention is therefore to provide a displaydevice that properly compensates for variations in the threshold voltageof a drive element and prevents unwanted light emission from anelectro-optical element.

Means for Solving the Problems

A first aspect of the present invention is an electric current drivingtype display device including:

a plurality of pixel circuits arranged so as to correspond to respectiveintersections of a plurality of scanning lines and a plurality of datalines;

a scanning signal output circuit that selects a write-target pixelcircuit using the scanning line; and

a display signal output circuit that provides potentials according todisplay data to the data lines, wherein

each of the pixel circuits includes:

an electro-optical element provided between a first power supply wiringline and a second power supply wiring line;

a drive element provided in series with the electro-optical element andbetween the first power supply wiring line and the second power supplywiring line;

a capacitor having a first electrode connected to a control terminal ofthe drive element;

a first switching element provided between a second electrode of thecapacitor and the data line;

a second switching element provided between the second electrode of thecapacitor and a predetermined power supply wiring line;

a third switching element provided between the control terminal and onecurrent input/output terminal of the drive element; and

a fourth switching element having one terminal connected to a thirdpower supply wiring line and having an other terminal connected directlyor through the third switching element to the control terminal of thedrive element.

A second aspect of the present invention is the display device accordingto the first aspect of the present invention, wherein

a potential that brings the drive element into a conduction state isapplied to the third power supply wiring line.

A third aspect of the present invention is the display device accordingto the first aspect of the present invention, wherein

the fourth switching element is provided between the third power supplywiring line and the control terminal of the drive element.

A fourth aspect of the present invention is the display device accordingto the third aspect of the present invention, wherein

when writing to the pixel circuit,

during a first period, the first and fourth switching elements arecontrolled to a conduction state and the second and third switchingelements are controlled to a non-conduction state,

then, during a second period, the fourth switching element is controlledto a non-conduction state and the third switching element is controlledto a conduction state, and

then, during a third period, the first and third switching elements arecontrolled to a non-conduction state and the second switching element iscontrolled to a conduction state.

A fifth aspect of the present invention is the display device accordingto the first aspect of the present invention, wherein the fourthswitching element is provided between the third power supply wiring lineand the current input/output terminal of the drive element, the terminalbeing connected to the third switching element.

A sixth aspect of the present invention is the display device accordingto the fifth aspect of the present invention, wherein

when writing to the pixel circuit,

during a first period, the first, third, and fourth switching elementsare controlled to a conduction state and the second switching element iscontrolled to a non-conduction state,

then, during a second period, the fourth switching element is controlledto a non-conduction state, and

then, during a third period, the first and third switching elements arecontrolled to a non-conduction state and the second switching element iscontrolled to a conduction state.

A seventh aspect of the present invention is the display deviceaccording to the first aspect of the present invention, wherein thesecond switching element is provided between the first power supplywiring line and the second electrode of the capacitor.

An eighth aspect of the present invention is the display deviceaccording to the seventh aspect of the present invention, wherein

a control terminal of the fourth switching element is connected to thethird power supply wiring line, and

a potential of the third power supply wiring line is switched between apotential that brings the drive element into a conduction state and apotential that brings the fourth switching element into a non-conductionstate.

A ninth aspect of the present invention is the display device accordingto the first aspect of the present invention, wherein the secondswitching element is provided between the third power supply wiring lineand the second electrode of the capacitor.

A tenth aspect of the present invention is the display device accordingto the ninth aspect of the present invention, wherein a potential of thethird power supply wiring line is configured to be controllable.

An eleventh aspect of the preset invention is the display deviceaccording to the first aspect of the present invention, wherein each ofthe pixel circuits further includes a fifth switching element providedbetween the drive element and the electro-optical element.

A twelfth aspect of the present invention is the display deviceaccording to the first aspect of the present invention, wherein whenwriting to the pixel circuit, a potential of the second power supplywiring line is controlled such that an applied voltage to theelectro-optical element is lower than a light-emission thresholdvoltage.

A thirteenth aspect of the present invention is the display deviceaccording to the first aspect of the present invention, wherein theelectro-optical element includes an organic EL element.

A fourteenth aspect of the present invention is the display deviceaccording to the first aspect of the present invention, wherein thedrive element and all of the switching elements in the pixel circuitinclude thin-film transistors.

A fifteenth aspect of the present invention is the display deviceaccording to the fourteenth aspect of the present invention, wherein thedrive element and all of the switching elements in the pixel circuitinclude thin-film transistors of a same channel type.

EFFECT OF THE INVENTION

According to the first or second aspect of the present invention, byapplying a potential that brings the drive element into a conductionstate to the third power supply wiring line and controlling the fourthswitching element (or the third and fourth switching elements) to aconduction state, a potential of the third power supply wiring line isprovided to the control terminal of the drive element and regardless ofa previous state of the pixel circuit, the drive element can be surelyset to a conduction state. Thus, when the third switching element iscontrolled to a conduction state, the drive element is reliably set to athreshold state (a state in which a threshold voltage is applied) andthus variations in the threshold voltage of the drive element can beproperly compensated for.

According to the third aspect of the present invention, since the fourthswitching element is provided between the third power supply wiring lineand the control terminal of the drive element, by controlling the fourthswitching element to a conduction state, a potential of the third powersupply wiring line can be provided to the control terminal of the driveelement.

According to the fourth aspect of the present invention, during thefirst period, a potential of the third power supply wiring line isprovided to the first electrode of the capacitor, a potential accordingto display data (hereinafter, also referred to as a data potential) isprovided to the second electrode of the capacitor, and a differencebetween these two potentials is held in the capacitor. During the secondperiod, the potential of the first electrode of the capacitor changesuntil the drive element goes into a threshold state, and accordingly,the potential difference held in the capacitor is changed to adifference between the data potential and the threshold voltage of thedrive element. During the third period, with the capacitor holding theabove-described potential difference, the potential of the secondelectrode of the capacitor is changed from the data potential to apotential of a predetermined power supply wiring line. Thus, the controlterminal potential of the drive element after that is a potentialobtained by adding a difference between the potential of thepredetermined power supply wiring line and the data potential to apotential at which the drive element goes into a threshold state.Therefore, even when the threshold voltage is different, if the datapotential is the same, then the amount of current flowing through thedrive element is the same. In this manner, variations in the thresholdvoltage of the drive element can be compensated for.

According to the fifth aspect of the present invention, since the fourthswitching element is provided between the third power supply wiring lineand the current input/output terminal of the drive element and theterminal is connected to the third switching element, by controllingboth the third and fourth switching elements to a conduction state, apotential of the third power supply wiring line can be provided to thecontrol terminal of the drive element. Also, since the control terminalof the drive element is connected to the third power supply wiring linethrough the third and fourth switching elements, the number of switchingelements connected to the control terminal of the drive element issmaller than the case in which the control terminal of the drive elementis connected to the third power supply wiring line through the fourthswitching element. Therefore, the control terminal potential of thedrive element is less likely to fluctuate due to a less amount ofleakage current flowing through a switching element. Thus, the luminanceof the electro-optical element can be properly kept and display qualitycan be enhanced.

According to the sixth aspect of the present invention, during the firstperiod, a potential of the third power supply wiring line is provided tothe first electrode of the capacitor, a data potential is provided tothe second electrode of the capacitor, and a difference between thesetwo potentials is held in the capacitor. During the second period, thepotential of the first electrode of the capacitor changes until thedrive element goes into a threshold state, and accordingly, thepotential difference held in the capacitor is changed to a differencebetween the data potential and the threshold voltage of the driveelement. During the third period, with the capacitor holding theabove-described potential difference, the potential of the secondelectrode of the capacitor is changed from the data potential to apotential of a predetermined power supply wiring line. Thus, the controlterminal potential of the drive element after that is a potentialobtained by adding a difference between the potential of thepredetermined power supply wiring line and the data potential to apotential at which the drive element goes into a threshold state.Therefore, even when the threshold voltage is different, if the datapotential is the same, then the amount of current flowing through thedrive element is the same. In this manner, variations in the thresholdvoltage of the drive element can be compensated for.

According to the seventh aspect of the present invention, by controllingthe second switching element to a conduction state, a potential of thefirst power supply wiring line can be provided to the second electrodeof the capacitor. Hence, the potential of the control terminal of thedrive element which is connected to the first electrode of the capacitorcan be kept at a level according to display data.

According to the eighth aspect of the present invention, bydiode-connecting the fourth switching element to the third power supplywiring line and switching the potential of the third power supply wiringline between predetermined levels, the fourth switching element can beswitched to a conduction state and a non-conduction state and the driveelement can be set to a conduction state. Accordingly, since a wiringline that controls the fourth switching element becomes unnecessary, thecircuit size of the display device can be reduced.

According to the ninth aspect of the present invention, by controllingthe second switching element to a conduction state, a potential of thethird power supply wiring line can be provided to the second electrodeof the capacitor. Accordingly, the potential of the control terminal ofthe drive element which is connected to the first electrode of thecapacitor can be kept at a level according to display data.

According to the tenth aspect of the present invention, since thecontrol terminal potential of the drive element increases or decreasesaccording to a difference between a potential of the third power supplywiring line and a data potential, by controlling the potential of thethird power supply wiring line, the luminance of all electro-opticalelements can be uniformly adjusted. Accordingly, only by adding a smallamount of circuit, without changing display data, a peak luminanceadjustment can be easily performed.

According to the eleventh aspect of the present invention, bycontrolling the fifth switching element to a non-conduction state whenwriting to the pixel circuit, a current flowing through theelectro-optical element from the drive element can be interrupted. Thus,the drive element can be properly set to a threshold state and unwantedlight emission from the electro-optical element can be prevented.

According to the twelfth aspect of the present invention, by controllingthe potential of the second power supply wiring line when writing to thepixel circuit, without providing a switching element between the firstpower supply wiring line and the second power supply wiring line, acurrent can be prevented from flowing through the electro-opticalelement. Thus, with a less amount of circuit, the drive element can beproperly set to a threshold state and unwanted light emission from theelectro-optical element can be prevented.

According to the thirteenth aspect of the present invention, an organicEL display that properly compensates for variations in the thresholdvoltage of the drive element can be obtained.

According to the fourteenth aspect of the present invention, byconfiguring the drive element and all switching elements in the pixelcircuit using thin-film transistors, the pixel circuit can be easilyfabricated with high precision.

According to the fifteenth aspect of the present invention, byconfiguring the drive element and all switching elements in the pixelcircuit using transistors of the same channel type, all transistors canbe fabricated using the same masks and by the same process, enabling tolower the cost of the display device. In addition, since transistors ofthe same channel type can be arranged closer to each other thantransistors of different channel types, a saved pixel circuit area canbe utilized for other purposes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a display deviceaccording to first to seventh (excluding fourth) embodiments of thepresent invention.

FIG. 2 is a circuit diagram of a pixel circuit included in a displaydevice according to the first embodiment of the present invention.

FIG. 3 is a timing chart of the pixel circuit shown in FIG. 2.

FIG. 4 is a circuit diagram of a pixel circuit included in a displaydevice according to the second embodiment of the present invention.

FIG. 5 is a timing chart of the pixel circuit shown in FIG. 4.

FIG. 6 is a circuit diagram of a pixel circuit included in a displaydevice according to the third embodiment of the present invention.

FIG. 7 is a timing chart of the pixel circuit shown in FIG. 6.

FIG. 8 is a block diagram showing a configuration of a display deviceaccording to the fourth embodiment of the present invention.

FIG. 9 is a circuit diagram of a pixel circuit included in the displaydevice according to the fourth embodiment of the present invention.

FIG. 10 is a timing chart of the pixel circuit shown in FIG. 9.

FIG. 11 is a circuit diagram of a pixel circuit included in a displaydevice according to the fifth embodiment of the present invention.

FIG. 12 is a timing chart of the pixel circuit shown in FIG. 11.

FIG. 13 is a circuit diagram of a pixel circuit included in a displaydevice according to the sixth embodiment of the present invention.

FIG. 14 is a timing chart of the pixel circuit shown in FIG. 13.

FIG. 15 is a circuit diagram of a pixel circuit included in a displaydevice according to the seventh embodiment of the present invention.

FIG. 16 is a timing chart of the pixel circuit shown in FIG. 15.

FIG. 17 is a circuit diagram of a pixel circuit (first example) includedin a conventional display device.

FIG. 18 is a timing chart of the pixel circuit shown in FIG. 17.

FIG. 19 is a circuit diagram of a pixel circuit (second example)included in a conventional display device.

FIG. 20 is a timing chart of the pixel circuit shown in FIG. 19.

FIG. 21 is a circuit diagram of a pixel circuit (third example) includedin a conventional display device.

DESCRIPTION OF THE REFERENCE NUMERALS

-   -   10 and 40: DISPLAY DEVICE    -   11: DISPLAY CONTROL CIRCUIT    -   12: GATE DRIVER CIRCUIT    -   13: SOURCE DRIVER CIRCUIT    -   14: REFERENCE SUPPLY ADJUSTMENT CIRCUIT    -   21: SHIFT REGISTER    -   22: REGISTER    -   23: LATCH CIRCUIT    -   24: D/A CONVERTER    -   48: REFERENCE POTENTIAL CONTROL CIRCUIT    -   100, 200, 300, 400, 500, 600, and 700: PIXEL CIRCUIT    -   110, 210, 310, 410, 510, 610, and 710: DRIVING TFT    -   111 to 115, 211 to 214, 311 to 315, 411 to 415, 511 to 515, 611        to 615, and 711 to 715: SWITCHING TFT    -   120, 220, 320, 420, 520, 620, and 720: CAPACITOR    -   130, 230, 330, 430, 530, 630, and 730: ORGANIC EL ELEMENT    -   Vp: POWER SUPPLY WIRING LINE    -   Vs: REFERENCE SUPPLY WIRING LINE    -   Vcom: COMMON CATHODE    -   CAi: CATHODE WIRING LINE    -   Wi, Ri, and Ei: CONTROL LINE    -   Gi: SCANNING LINE    -   Sj: DATA LINE

BEST MODE FOR CARRYING OUT THE INVENTION

Display devices according to first to seventh embodiments of the presentinvention will be described below with reference to FIGS. 1 to 16. Adisplay device according to each embodiment includes a pixel circuitincluding an electro-optical element, a drive element, a capacitor, anda plurality of switching elements. The pixel circuit includes an organicEL element as the electro-optical element; and a driving TFT andswitching TFTs which are composed of CG silicon TFTs as the driveelement and the switching elements. Note that the drive element and theswitching elements can be composed of, for example, amorphous siliconTFTs or low-temperature polysilicon TFTs, in addition to CS siliconTFTs. By composing the drive element and the switching elements usingTFTs, the pixel circuit can be easily fabricated with high precision.

The configuration of a CS silicon TFT is disclosed in Inukai, and sevenothers, “4.0-in. TFT-OLED Displays and a Novel Digital Driving Method”,SID'00 Digest, pp. 924-927. A CG silicon TFT fabrication process isdisclosed in Takayama, and five others, “Continuous Grain SiliconTechnology and Its Applications for Active Matrix Display”, AMD LCD2000, pp. 25-28. The configuration of an organic EL element is disclosedin Friend, “Polymer Light-Emitting Diodes for use in Flat PanelDisplay”, AM-LCD'01, pp. 211-214. Hence, description of these matters isomitted.

FIG. 1 is a block diagram showing a configuration of a display deviceaccording to the first to seventh (excluding the fourth) embodiments ofthe present invention. A display device 10 shown in FIG. 1 includes aplurality of pixel circuits Aij (i is an integer greater than or equalto 1 and less than or equal to n and j is an integer greater than orequal to 1 and less than or equal to m); a display control circuit 11; agate driver circuit 12; a source driver circuit 13; and a referencesupply adjustment circuit 14. In the display device 10, a plurality ofscanning lines Gi parallel to one another and a plurality of data linesSj parallel to one another and orthogonal to the scanning lines Gi areprovided. The pixel circuits Aij are arranged in a matrix form so as tocorrespond to respective intersections of the scanning lines Gi and thedata lines Sj.

In addition to this, in the display device 10, a plurality of controllines (Wi, Ri, etc., which are not shown) parallel to one another arearranged in parallel to the scanning lines Gi. The scanning lines Gi andthe control lines are connected to the gate driver circuit 12 and thedata lines Sj are connected to the source driver circuit 13. The gatedriver circuit 12 and the source driver circuit 13 function as drivecircuits for the pixel circuits Aij.

The display control circuit 11 outputs a timing signal OE, a start pulseYI, and a clock YCK to the gate driver circuit 12, outputs a start pulseSP, a clock CLK, display data DA, and a latch pulse LP to the sourcedriver circuit 13, and outputs a voltage control signal PDA to thereference supply adjustment circuit 14.

The gate driver circuit 12 includes a shift register circuit, a logicoperation circuit, and a buffer (none of which are shown). The shiftregister circuit sequentially transfers the start pulse Y1 insynchronization with the clock YCK. The logic operation circuit performsa logic operation between a pulse outputted from each stage of the shiftregister circuit and the timing signal OE. An output from the logicoperation circuit is provided to a corresponding scanning line Gi,corresponding control lines Wi, Ri, etc., through the buffer. As such,the gate driver circuit 12 functions as a scanning signal output circuitthat selects a write-target pixel circuit using a corresponding scanningline Gi.

The source driver circuit 13 includes an m-bit shift register 21, aregister 22, a latch circuit 23, and m D/A converters 24. The shiftregister 21 includes m cascade-connected one-bit registers. The shiftregister 21 sequentially transfers the start pulse SP in synchronizationwith the clock CLK and outputs timing pulses DLP from the registers ofthe respective stages. In accordance with output timing of the timingpulses DLP, the display data DA is supplied to the register 22. Theregister 22 stores the display data DA according to the timing pulsesDLP. When an amount of the display data DA corresponding to one row isstored in the register 22, the display control circuit 11 outputs thelatch pulse LP to the latch circuit 23. When the latch circuit 23receives the latch pulse LP, the latch circuit 23 holds the display datastored in the register 22. The D/A converters 24 are provided to thedata lines Sj on a one-to-one basis. The D/A converters 24 convert thedisplay data held in the latch circuit 23 to analog signal voltages andprovide the analog signal voltages to corresponding data lines Sj. Assuch, the source driver circuit 13 functions as a display signal outputcircuit that provides potentials according to display data to the datalines Sj.

Note that in order for the display device 10 to achieve reduction insize and cost, it is desirable that all or part of the gate drivercircuit 12 and the source driver circuit 13 be formed on the samesubstrate as that for the pixel circuits Aij, using CG silicon TFTs orpolycrystal silicon TFTs.

The reference supply adjustment circuit 14 adjusts the level of apotential (hereinafter, referred to as the reference potential Vstd) tobe applied to a reference supply wiring line Vs, based on the voltagecontrol signal PDA. All of the pixel circuits Aij are connected to thereference supply wiring line Vs and receive a supply of the referencepotential Vstd from the reference supply adjustment circuit 14. Thoughnot shown in FIG. 1, in a region where the pixel circuits Aij arearranged, a power supply wiring line Vp and a common cathode Vcom (or acathode wiring line CAi) are arranged to supply a supply voltage to thepixel circuits Aij.

The pixel circuits Aij included in the display device according to eachembodiment will be described in detail below. In the followingdescription, a high-level potential provided to a gate terminal of aswitching TFT is referred to as GH and a low-level potential is referredto as GL. Also, although in the following description the channel typeof each TFT is fixedly determined, provided that an appropriate controlsignal can be supplied to a gate terminal of each TFT, each TFT may beof either a p-channel type or an n channel type.

FIRST EMBODIMENT

FIG. 2 is a circuit diagram of a pixel circuit included in a displaydevice according to the first embodiment of the present invention, Apixel circuit 100 shown in FIG. 2 includes a driving TFT 110, switchingTFTs 111 to 115, a capacitor 120, and an organic EL element 130. Theswitching TFTs 111 and 114 are of an n-channel type and other TFTs areof a p-channel type.

The pixel circuit 100 is connected to a power supply wiring line Vp, areference supply wiring line Vs, a common cathode Vcom, a scanning lineGi, control lines Wi and Ri, and a data line Sj. Of them, to the powersupply wiring line Vp (first power supply wiring line) and the commoncathode Vcom (second power supply wiring line) are respectively appliedfixed potentials VDD and VSS and to the reference supply wiring line Vs(third power supply wiring line) is applied a reference potential Vstdobtained by the reference supply adjustment circuit 14. The commoncathode Vcom serves as a common electrode for all organic EL elements130 in the display device.

In the pixel circuit 100, on a path connecting the power supply wiringline Vp to the common cathode Vcom, in order from the side of the powersupply wiring line Vp, the driving TFT 110, the switching TFT 115, andthe organic EL element 130 are provided in series. One electrode of thecapacitor 120 is connected to a gate terminal of the driving TFT 110.Between the other electrode of the capacitor 120 and the data line Sj,the switching TFT 111 is provided. Hereinafter, a connection pointbetween the driving TFT 110 and the capacitor 120 is referred to as Aand a connection point between the capacitor 120 and the switching TFT111 is referred to as B. The switching TFT 112 is provided between theconnection point B and the power supply wiring line Vp, the switchingTFT 113 is provided between the connection point A and a drain terminalof the driving TFT 110, and the switching TFT 114 is provided betweenthe connection point A and the reference supply wiring line Vs.

Gate terminals of the respective switching TFTs 111, 112, and 115 areconnected to the scanning line Gi, a gate terminal of the switching TFT113 is connected to the control line Wi, and a gate terminal of theswitching TFT 114 is connected to the control line Ri. The potentials ofthe scanning line Gi and the control lines Wi and Ri are controlled bythe gate driver circuit 12 and the potential of the data line Sj iscontrolled by the source driver circuit 13.

FIG. 3 is a timing chart of the pixel circuit 100. FIG. 3 shows changesin potentials applied to the scanning line Gi, the control lines Wi andRi, and the data line Sj and changes in potentials at the connectionpoints A and B. In FIG. 3, the period from time t0 to time t5corresponds to one horizontal scanning period. With reference to FIG. 3,the operation of the pixel circuit 100 will be described below.

Before time t0, the potentials of the scanning line Gi and the controlline Ri are controlled to CL (low level) the potential of the controlline Wi is controlled to GH (high level), and the potential of the dataline Sj is controlled to a level according to display data for the lasttime (display data written to a pixel circuit in an adjacent upper row).Thus, the switching TFTs 112 and 115 are in a conduction state and theswitching TFTs 111, 113, and 114 are in a non-conduction state. Thepotential at the connection point A is a potential according to displaydata written to the pixel circuit 100 last time and the potential at theconnection point B is VDD.

When at time t0 the potential of the scanning line Gi is changed to GH,the switching TFT 111 is changed to a conduction state and the switchingTFTs 112 and 115 are changed to a non-conduction state. Since, while thepotential of the scanning line Gi is GH (during the period from time t0to time t5), the switching TFT 115 is in a non-conduction state, acurrent does not flow through the organic EL element 130 and thus theorganic EL element 130 does not emit light.

While the potential of the scanning line Gi is GH, the potential of thedata line Sj is controlled to a potential of a level according todisplay data for this time (hereinafter, referred to as the datapotential Vdata). During this period, the connection point B isconnected to the data line Sj through the switching TFT 111, and thus,the potential at the connection point B is Vdata. During the period fromtime t0 to time t1, the switching TFTs 113 and 114 are in anon-conduction state, and thus, when the potential at the connectionpoint B is changed from VDD to Vdata, the potential at the connectionpoint A is also changed by the same amount (Vdata−VDD).

Then, when at time t1 the potential of the control line Ri is changed toGH, the switching TFT 114 is changed to a conduction state. Thus, theconnection point A is connected to the reference supply wiring line Vsthrough the switching TFT 114 and thus the potential at the connectionpoint A is changed to Vstd. At this time, since the connection point Bis connected to the data line Sj through the switching TFT 111, evenwhen the potential at the connection point A is changed, the potentialat the connection point B remains at Vdata.

The reference potential Vstd of the reference supply wiring line Vs isdetermined such that the driving TFT 110 goes into a conduction statewhen the reference potential Vstd is applied to the gate terminal.Hence, after time t1, the driving TFT 110 is surely in a conductionstate. Note that even when the driving TFT 110 goes into a conductionstate, while the switching TFT 115 is in a non-conduction state, acurrent does not flow through the organic EL element 130 and thus theorganic EL element 130 does not emit light.

Then, when at time t2 the potential of the control line Ri is changed toGL, the switching TFT 114 is changed to a non-conduction state. Thus,the connection point A is disconnected from the reference supply wiringline Vs and thus the potential at the connection point A is fixed. Atthis timer a potential difference (Vstd−Vdata) between the connectionpoints A and B is held in the capacitor 120.

Then, when at time t3 the potential of the control line Wi is changed toGL, the switching TFT 113 is changed to a conduction state. Thus, thegate and drain terminals of the driving TFT 110 are short-circuited,whereby the driving TFT 110 forms a diode connection. During the periodfrom time t1 to time t2, the reference potential Vstd is applied to theconnection point A and after time t2 too, the potential at theconnection point A is kept at Vstd by the capacitor 120. Therefore,after time t3 too, the driving TFT 110 is surely in a conduction state.

A current flows into the connection point A from the power supply wiringline Vp through the driving TFT 110 and the switching TFT 113, and thepotential at the connection point A (gate terminal potential of thedriving TFT 110) rises while the driving TFT 110 is in a conductionstate. The driving TFT 110 is changed to a non-conduction state when thegate-source voltage becomes a threshold voltage Vth (negative value)(i.e., the potential at the connection point A becomes (VDD+Vth)).Therefore, the potential at the connection point A rises to (VDD+Vth)and the driving TFT 110 goes into a threshold state (a state in which athreshold voltage is applied between the gate and the source).

Then, when at time t4 the potential of the control line Wi is changed toGH, the switching TFT 113 is changed to a non-conduction state. At thistime, a potential difference (VDD+Vth−Vdata) between the connectionpoints A and B is held in the capacitor 120.

Then, when at time t5 the potential of the scanning line Gi is changedto GL, the switching TFTs 112 and 115 are changed to a conduction stateand the switching TFT 111 is changed to a non-conduction state. Thus,the connection point B is disconnected from the data line Sj andconnected to the power supply wiring line Vp through the switching TFT112. Hence, the potential at the connection point B is changed fromVdata to VDD and accordingly the potential at the connection point A isalso changed by the same amount (VDD−Vdata; hereinafter, referred to asVB) and becomes (VDD+Vth+VB).

After time t5, the switching TFT 115 is in a conduction state and thus acurrent flows through the organic EL element 130 from the power supplywiring line Vp through the driving TFT 110 and the switching TFT 115.Although the amount of current flowing through the driving TFT 110increases or decreases depending on the gate terminal potential(VDD+Vth+VB), even when the threshold voltage Vth is different, if thepotential difference VB (=VDD−Vdata) is the same, then the amount ofcurrent is the same. Therefore, regardless of the value of the thresholdvoltage Vth of the driving TFT 110, a current of an amount according tothe data potential Vdata flows through the organic EL element 130 andthus the organic EL element 130 emits light with a specified luminance.

In the above-described operation, after the switching TFT 114 is changedto a non-conduction state at time t2, at time t3 the switching TFT 113is changed to a conduction state. Thus, a current is prevented fromflowing into the reference supply wiring line Vs from the power supplywiring line Vp through the driving TFT 110 and the switching TFTs 113and 114 and thus the potential of the reference supply wiring line Vscan be kept stable. In addition, since the potential difference held inthe capacitor 120 at time t2 does not change, variations in thresholdvoltage can be precisely compensated for.

In the above-described operation, after the switching TFT 113 is changedto a non-conduction state at time t4, at time t5 the switching TFT 111is changed to a non-conduction state and the switching TFT 112 ischanged to a conduction state. Thus, a current is prevented from flowinginto the connection point A from the power supply wiring line Vp throughthe driving TFT 110 and the switching TFT 113 and thus the gate terminalpotential of the driving TFT 110 can be kept precisely.

As described above, according to the display device according to thepresent embodiment, by applying the reference potential Vstd that bringsthe driving TFT 110 into a conduction state to the reference supplywiring line Vs and controlling the switching TFT 114 to a conductionstate, the reference potential Vstd is provided to the gate terminal ofthe driving TFT 110 and regardless of a previous state of the pixelcircuit, the driving TFT 110 can be surely set to a conduction state.

Therefore, when, after that, the switching TFT 113 is controlled to aconduction state and the switching TFT 115 is controlled to anon-conduction state, the driving TFT 110 can be reliably set to athreshold state and the current flowing through the organic EL element130 from the driving TFT 110 can be interrupted. Thus, the driving TFT110 can be properly set to a threshold state and unwanted light emissionfrom the organic EL element 130 can be prevented. When unwanted lightemission can be prevented, the contrast of a display screen improves andthe lifetime of the organic EL element 130 is also extended.

In order to set the driving TFT 110 of a p-channel type to a conductionstate, the reference potential Vstd to be applied to the gate terminalneeds to be lowered than the source terminal potential of the drivingTFT 110 by an amount greater than or equal to the threshold voltage Vth.However, if the reference potential Vstd is lowered too much, it takestime for the driving TFT 110 to go into a threshold state, and thus aprocess of compensating for variations in the threshold voltage of thedriving TFT 110 may not be completed during one horizontal scanningperiod. Hence, it is desirable that the reference potential Vstd be apotential as close to (VDD+Vth) as possible, provided that the referencepotential Vstd satisfies the condition that the driving TFT 110 goesinto a conduction state when the reference potential Vstd is provided tothe gate terminal.

Since the pixel circuit 100 operates based on the reference potentialVstd provided externally, the level of the reference potential Vstd canbe freely set using the reference supply adjustment circuit 14, etc.Therefore, according to the display device according to the presentembodiment, by using the reference potential Vstd close to (VDD+Vth),variations in the threshold voltage of the driving TFT can becompensated for in a short period of time.

Before the driving TFT 110 is brought into a threshold state, apotential difference (Vstd-Vdata) is held in the capacitor 120. Thispotential difference is the same for all pixel circuits. Thus, even ifthe driving TFT 110 cannot be completely set to a threshold state,variations in the luminance of the organic EL element can be made small.

SECOND EMBODIMENT

FIG. 4 is a circuit diagram of a pixel circuit included in a displaydevice according to the second embodiment of the present invention. Apixel circuit 200 shown in FIG. 4 includes a driving TFT 210, switchingTFTs 211 to 214, a capacitor 220, and an organic EL element 230. Theswitching TFTs 211 and 214 are of an n-channel type and other TFTs areof a p-channel type.

The pixel circuit 200 is obtained by making a change to the pixelcircuit 100 (FIG. 2) according to the first embodiment such that theswitching TFT 115 is eliminated and a cathode terminal of the organic ELelement 130 is connected to a cathode wiring line CAi (second powersupply wiring line). In the pixel circuit 200, on a path connecting apower supply wiring line Vp to the cathode wiring line CAi, in orderfrom the side of the power supply wiring line Vp, the driving TFT 210and the organic EL element 230 are provided in series. Except for theabove points, the configuration of the pixel circuit 200 is the same asthat of the pixel circuit 100. The potential of the cathode wiring lineCAi is controlled by a power supply switching circuit (not shown)included in the display device 10.

FIG. 5 is a timing chart of the pixel circuit 200. FIG. 5 shows changesin potentials applied to a scanning line Gi, control lines Wi and Ri,the cathode wiring line CAi, and a data line Sj and changes inpotentials at connection points A and B. In FIG. 5, the period from timet0 to time t5 corresponds to one horizontal scanning period.

As shown in FIG. 5, the potential of the cathode wiring line CAi iscontrolled to a predetermined level Vch during the period from time t0to time t5 and controlled to VSS during other times. The potential Vchis determined such that when a potential VDD is applied to one end of acircuit made by connecting the driving TFT 210 to the organic EL element230 in series and the potential Vch is applied to the other end, anapplied voltage to the organic EL element 230 is lower than alight-emission threshold voltage of the organic EL element 230. Hence,while the potential of the cathode wiring line CAi is Vch (during theperiod from time t0 to time t5), a current contributing to lightemission does not flow through the organic EL element 230 and thus theorganic EL element 230 does not emit light. Except for the above points,the operation of the pixel circuit 200 is the same as that of the pixelcircuit 100.

As described above, in the display device according to the presentembodiment, when writing to the pixel circuit, the potential of thecathode wiring line CAi is controlled to a level at which a current doesnot flow through the organic EL element 230. Therefore, even withoutproviding a switching TFT on a path connecting the power supply wiringline Vp to the cathode wiring line CAi, the same effect (of properlycompensating for variations in the threshold voltage of the driving TFTin a short period of time and preventing unwanted light emission fromthe organic EL element) as that obtained in the first embodiment can beobtained.

THIRD EMBODIMENT

FIG. 6 is a circuit diagram of a pixel circuit included in a displaydevice according to the third embodiment of the present invention. Apixel circuit 300 shown in FIG. 6 includes a driving TFT 310, switchingTFTs 311 to 315, a capacitor 320, and an organic EL element 330. All ofthe TFTs included in the pixel circuit 300 are of a p-channel type.

The pixel circuit 300 is obtained by making a change to the pixelcircuit 100 (FIG. 2) according to the first embodiment such that theTFTs of an n-channel type are changed to TFT of a p-channel type and agate terminal of each TFT is connected to an appropriate signal line. Inthe pixel circuit 300, gate terminals of the respective switching TFTs311 and 313 are connected to a scanning line Gi, gate terminals of therespective switching TFTs 312 and 315 are connected to a control lineEi, and a gate terminal of the switching TFT 314 is connected to acontrol line Ri. Except for the above points, the configuration of thepixel circuit 300 is the same as that of the pixel circuit 100. Thepotential of the control line Ei is controlled by the gate drivercircuit 12.

FIG. 7 is a timing chart of the pixel circuit 300. FIG. 7 shows changesin potentials applied to the scanning line Gi, the control lines Ei andRi, and a data line Sj and changes in potentials at connection points Aand B. In FIG. 7, the period from time t0 to time t4 corresponds to onehorizontal scanning period. With reference to FIG. 7, the operation ofthe pixel circuit 300 will be described below.

Before time t0, the potentials of the scanning line Gi and the controlline Ri are controlled to GH, the potential of the control line Ei iscontrolled to GL, and the potential of the data line Sj is controlled toa level according to display data for the last time. Thus, the switchingTFTs 312 and 315 are in a conduction state and the switching TFTs 311,313, and 314 are in a non-conduction state. The potential at theconnection point A is a potential according to display data written tothe pixel circuit 300 last time and the potential at the connectionpoint B is VDD.

When at time t0 the potential of the control line Ei is changed to GH,the switching TFTs 312 and 315 are changed to a non-conduction state.Since, while the potential of the control line Ei is GH (during theperiod from time t0 to time t4), the switching TFT 315 is in anon-conduction state, a current does not flow through the organic ELelement 330 and thus the organic EL element 330 does not emit light.

While the potential of the control line Ei is GH, the potential of thedata line Sj is controlled to a data potential Vdata. Since, during theperiod from time t0 to time t1, the connection points A and B aredisconnected from wiring lines to which potentials are applied, thepotentials at the connection points A and B become undefined (in fact,the potentials do not change from the level at time t0).

Then, when at time t1 the potentials of the scanning line Gi and thecontrol line Ri are changed to GL, the switching TFTs 311, 313, and 314are changed to a conduction state. Thus, the connection point B isconnected to the data line Sj through the switching TFT 311, and thus,the potential at the connection point B is changed to Vdata. Since theconnection point A is connected to a reference supply wiring line Vsthrough the switching TFT 314, the potential at the connection point Ais changed to Vstd. The reference potential Vstd of the reference supplywiring line Vs is, as with the first embodiment, determined such thatthe driving TFT 310 goes into a conduction state when the referencepotential Vstd is applied to the gate terminal. Hence, after time t1,the driving TFT 310 is surely in a conduction state. Note that even whenthe driving TFT 310 goes into a conduction state, while the switchingTFT 315 is in a non-conduction state, a current does not flow throughthe organic EL element 330 and thus the organic EL element 330 does notemit light.

On the other hand, when the switching TFT 313 goes into a conductionstate, the gate and drain terminals of the driving TFT 310 areshort-circuited, whereby the driving TFT 310 forms a diode connection.Hence, a current flows into the connection point A from a power supplywiring line Vp through the driving TFT 310 and the switching TFT 313 andthus the potential at the connection point A rises by an amountcorresponding to the current flown into. Accordingly, the potential atthe connection point A becomes, strictly speaking, a potential (Vstd+α)which is a little higher than the reference potential Vstd.

Then, when at time t2 the potential of the control line Ri is changed toGH, the switching TFT 314 is changed to a non-conduction state. Thus,the current flowing through the connection point A from the referencesupply wiring line Vs through the switching TFT 314 is interrupted.Instead of this, a current flows into the connection point A from thepower supply wiring line Vp through the driving TFT 310 and theswitching TFT 313, and the potential at the connection point A (the gateterminal potential of the driving TFT 310) rises while the driving TFT310 is in a conduction state. The driving TFT 310 is changed to anon-conduction state when the gate-source voltage becomes a thresholdvoltage Vth (negative value) (i.e., the potential at the connectionpoint A becomes (VDD+Vth)). Therefore, the potential at the connectionpoint A rises to (VDD+Vth) and the driving TFT 310 goes into a thresholdstate.

Then, when at time t3 the potential of the scanning line Gi is changedto GH, the switching TFTs 311 and 313 are changed to a non-conductionstate. At this time, a potential difference (VDD+Vth−Vdata) betweenconnection points A and B is held in the capacitor 320.

Then, when at time t4 the potential of the control line Ei is changed toGL, the switching TFTs 312 and 315 are changed to a conduction state.Thus, the connection point B is connected to the power supply wiringline Vp through the switching TFT 312. At this time, the potential atthe connection point B is changed from Vdata to VDD and accordingly thepotential at the connection point A is changed by the same amount(VDD−Vdata; hereinafter, referred to as VB) and becomes (VDD+Vth+VB).

After time t4, the switching TFT 315 is in a conduction state and thus acurrent flows through the organic EL element 330 from the power supplywiring line Vp through the driving TFT 310 and the switching TFT 315.Although the amount of current flowing through the driving TFT 310increases or decreases depending on the gate terminal potential(VDD+Vth+VB), even when the threshold voltage Vth is different, if thepotential difference VB (=VDD−Vdata) is the same, then the amount ofcurrent is the same. Therefore, regardless of the value of the thresholdvoltage Vth of the driving TFT 310, a current of an amount according tothe data potential Vdata flows through the organic EL element 330 andthus the organic EL element 330 emits light with a specified luminance.

As described above, in the pixel circuit 300, the driving TFT 310 andall of the switching TFTs 311 to 315 are composed of transistors of thesame channel type. Even the display device according to the presentembodiment including such a pixel circuit 300 can obtain the same effectas that obtained in the first embodiment by supplying an appropriatecontrol signal to the gate terminal of each TFT. In addition, sincetransistors of the same channel type can be fabricated using the samemasks and by the same process, the cost of the display device can bereduced. Also, since transistors of the same channel type can bearranged closer to each other than transistors of different channeltypes, a saved pixel circuit area can be utilized for other purposes.

FOURTH EMBODIMENT

FIG. 8 is a block diagram showing a configuration of a display deviceaccording to the fourth embodiment of the present invention. A displaydevice 40 shown in FIG. 8 is such that in the display device 10 shown inFIG. 1 the reference supply adjustment circuit 14 is replaced by areference potential control circuit 48. In the display device 40, inorder to supply a reference potential to pixel circuits Aij, instead ofthe reference supply wiring line Vs connected to all pixel circuits Aij,n control lines Ri connected to the respective rows of the pixelcircuits Aij are used.

The reference potential control circuit 48 adjusts the levels of twotypes of reference potential (hereinafter, referred to as Vsh and Vs1)based on a voltage control signal PDA. The reference potential controlcircuit 48 is connected to the n control lines Ri and individuallyswitches the potentials of the control lines Ri between Vsh and Vs1.

FIG. 9 is a circuit diagram of a pixel circuit included in the displaydevice according to the fourth embodiment of the present invention. Apixel circuit 400 shown in FIG. 9 includes a driving TFT 410, switchingTFTs 411 to 415, a capacitor 420, and an organic EL element 430. Theswitching TFT 411 is of an n-channel type and other TFTs are of ap-channel type.

The pixel circuit 400 is obtained by making a change to the pixelcircuit 100 (FIG. 2) according to the first embodiment such that theswitching TFT 114 is changed to a TFT of a p channel type and thechanged TFT is diode connected to a control line Ri. In the pixelcircuit 400, both gate and drain terminals of the switching TFT 414 areconnected to the control line Ri (third power supply wiring line).Except for the above points, the configuration of the pixel circuit 400is the same as that of the pixel circuit 100.

FIG. 10 is a timing chart of the pixel circuit 400. FIG. 10 showschanges in potentials applied to a scanning line Gi, control lines Wiand Ri, and a data line Sj and changes in potentials at connectionpoints A and B. In FIG. 10, the period from time t0 to time t5corresponds to one horizontal scanning period. With reference to FIG.10, differences in operation between the pixel circuit 400 and the pixelcircuit 100 will be described below.

As shown in FIG. 10, the potential of the control line Ri is controlledto Vs1 during the period from time t1 to time t2 and controlled to Vshduring other times. The reference potentials Vsh and Vs1 are determinedto satisfy conditions which will be described later.

When at time t1 the potential of the control line Ri is changed to Vs1,both a gate terminal potential and a drain terminal potential of theswitching TFT 414 are changed to Vs1. The switching TFT 414 of ap-channel type goes into a conduction state when the gate-source voltageis lower than a threshold voltage (i.e., when the potential Vs1 is lowerthan the potential at the connection point A by an amount greater thanor equal to the threshold voltage of the switching TFT 414).

When the switching TFT 414 goes into a conduction state, a current flowsout to the control line Ri from the connection point A through theswitching TFT 414 and the potential at the connection point A dropswhile the switching TFT 414 is in a conduction state. The switching TFT414 is changed to a non-conduction state when the gate-source voltagebecomes a threshold voltage Vth′ (negative voltage) (i.e., the potentialat the connection point A becomes (Vs1−Vth′)). Thus the potential at theconnection point A drops to (Vs1−Vth′). Furthermore, when the potentialat the connection point A at this time is lower than the source terminalpotential of the driving TFT 410 by an amount greater than or equal tothe threshold voltage Vth (negative value) (i.e., when Vs1−Vth′<VDD+Vthis satisfied), the driving TFT 410 goes into a conduction state.

Hence, the reference potential Vs1 is determined such that regardless ofa previous potential at the connection point A, when the referencepotential Vs1 is applied to the gate terminal of the switching TFT 414,the switching TFT 414 goes into a conduction state and furthermore thedriving TFT 410 goes into a conduction state. On the other hand, thereference potential Vsh is determined such that regardless of a previouspotential at the connection point A, when the reference potential Vsh isapplied to the gate terminal of the switching TFT 414, the switching TFT414 goes into a non-conduction state. By switching the potential of thecontrol line Ri between Vsh and Vs1 that satisfy these conditions, thedriving TFT 410 can be set to a conduction state using only one controlline.

As described above, in the pixel circuit 400, by switching the potentialof the control line Ei between Vsh and Vs1 with the switching TFT 414being diode-connected to the control line Ri, the switching TFT 414 isswitched to a conduction state and a non-conduction state and thedriving TFT 410 can be set to a conduction state. Accordingly, even withthe display device according to the present embodiment including thepixel circuit 400, the same effect as that obtained in the firstembodiment can be obtained. In addition, since a wiring line thatcontrols the switching TFT 414 becomes unnecessary, the circuit size ofthe display device can be reduced.

FIFTH EMBODIMENT

FIG. 11 is a circuit diagram of a pixel circuit included in a displaydevice according to the fifth embodiment of the present invention. Apixel circuit 500 shown in FIG. 11 includes a driving TFT 510, switchingTFTs 511 to 515, a capacitor 520, and an organic EL element 530. Theswitching TFTs 511 and 514 are of an n-channel type and other TFTs areof a p-channel type.

The pixel circuit 500 is obtained by making a change to the pixelcircuit 100 (FIG. 2) according to the first embodiment such that theswitching TFT 112 is connected to a reference supply wiring line Vs. Inthe pixel circuit 500, the switching TFT 512 is provided between aconnection point B and the reference supply wiring line Vs. Except forthe above points, the configuration of the pixel circuit 500 is the sameas that of the pixel circuit 100.

FIG. 12 is a timing chart of the pixel circuit 500. FIG. 12 showschanges in potentials applied to a scanning line Gi, control lines Wiand Ri, and a data line Sj and changes in potentials at connectionpoints A and B. In FIG. 12, the period from time t0 to time t5corresponds to one horizontal scanning period. With reference to FIG.12, differences in operation between the pixel circuit 500 and the pixelcircuit 100 will be described below.

As shown in FIG. 12, the pixel circuit 500 operates in the same manneras the pixel circuit 100 during the period from time t0 to time t5. Whenat time t5 the potential of the scanning line Gi is changed to GL, theswitching TFTs 512 and 515 are changed to a conduction state and theswitching TFT 511 is changed to a non-conduction state. Thus, theconnection point B is disconnected from the data line Sj and connectedto the reference supply wiring line Vs through the switching TFT 512.Hence, the potential at the connection point B is changed from Vdata toVstd and accordingly the potential at the connection point A is alsochanged by the same amount (Vstd Vdata; hereinafter, referred to as VC)and becomes (VDD+Vth+VC).

After time t5, the switching TFT 515 is in a conduction state and thus acurrent flows through the organic EL element 530 from a power supplywiring line Vp through the driving TFT 510 and the switching TFT 515.Although the amount of current flowing through the driving TFT 510increases or decreases depending on the gate terminal potential(VDD+Vth+VC), even when the threshold voltage Vth is different, if thepotential difference VC (=Vstd−Vdata) is the same, then the amount ofcurrent is the same. Therefore, regardless of the value of the thresholdvoltage Vth of the driving TFT 510, a current of an amount according tothe potential Vdata applied to the data line Sj flows through theorganic EL element 530 and thus the organic EL element 530 emits lightwith a specified luminance.

As described above, in the pixel circuit 500, the switching TFT 512 isprovided between the connection point B and the reference supply wiringline Vs. Even with the display device according to the presentembodiment including such a pixel circuit 500, the gate terminalpotential of the driving TFT 510 can be kept at a level according to thedata potential Vdata, and thus, the same effect as that obtained in thefirst embodiment can be obtained. In addition to this, according to thedisplay device according to the present embodiment, a peak luminanceadjustment for improving display quality can be easily performed, asshown below.

In order to perform a peak luminance adjustment in a conventionaldisplay device, there is a need, for example, to accumulate display datain a memory, etc., to determine a peak luminance, perform a conversionprocess according to the determined peak luminance on the display data,and provide a potential according to the converted display data to apixel circuit. However, to perform these processes, there is a need toadd a memory and an arithmetic circuit to a display control circuit or asource driver circuit and add a circuit that supports a peak luminanceadjustment to an output portion of the source driver circuit. Therefore,adding a peak luminance adjustment function to a conventional displaydevice greatly increases the cost and power consumption of the displaydevice.

In contrast, in the display device according to the present embodiment,since the gate terminal potential of the driving TFT 510 is (VDD+Vth+VC)and the potentials VDD and Vth have fixed values, the luminance of theorganic EL element 530 increases or decreases according to the potentialdifference VC (=Vstd−Vdata). Therefore, even without individuallychanging the data potential Vdata, by adjusting the reference potentialVstd according to the peak luminance by the reference supply adjustmentcircuit 14, the luminance of the organic EL element 530 can be uniformlyadjusted. In this case, a circuit does not need to be added to an outputportion of the source driver circuit. Thus, according to the displaydevice according to the present embodiment, only by adding a smallamount of circuit, without changing display data, a peak luminanceadjustment can be easily performed.

SIXTH EMBODIMENT

FIG. 13 is a circuit diagram of a pixel circuit included in a displaydevice according to the sixth embodiment of the present invention. Apixel circuit 600 shown in FIG. 13 includes a driving TFT 610, switchingTFTs 611 to 615, a capacitor 620, and an organic EL element 630. Theswitching TFTs 612, 614, and 615 are of a p-channel type and other TFTsare of an n-channel type.

The pixel circuit 600 is obtained by making a change to the pixelcircuit 500 (FIG. 11) according to the fifth embodiment such that thedriving TFT 510 and the switching TFT 513 are changed to TFT of ann-channel type, the switching TFT 514 is changed to a TFT of a p-channeltype, and the arrangement order of elements on a path connecting a powersupply wiring line Vp to a common cathode Vcom is changed. In the pixelcircuit 600, on the path connecting the power supply wiring line Vp tothe common cathode Vcom, in order from the side of the power supplywiring line Vp, the organic EL element 630, the switching TFT 615, andthe driving TFT 610 are provided in series. Except for the above points,the configuration of the pixel circuit 600 is the same as that of thepixel circuit 500.

FIG. 14 is a timing chart of the pixel circuit 600. FIG. 14 showschanges in potentials applied to a scanning line Gi, control lines Wiand Ri, and a data line Sj and changes in potentials at connectionpoints A and B. In FIG. 14, the period from time t0 to time t5corresponds to one horizontal scanning period. With reference to FIG.14, the operation of the pixel circuit 600 will be described below.

Before time t0, the potentials of the scanning line Gi and the controlline Wi are controlled to CL, the potential of the control line Ri iscontrolled to GH, and the potential of the data line Sj is controlled toa level according to display data for the last time. Hence, theswitching TFTs 612 and 615 are in a conduction state and the switchingTFTs 611, 613, and 614 are in a non-conduction state. The potential atthe connection point A is a potential according to data written to thepixel circuit 600 last time and the potential at the connection point Bis Vstd.

When at time t0 the potential of the scanning line Gi is changed to GH,the switching TFT 611 is changed to a conduction state and the switchingTFTs 612 and 615 are changed to a non-conduction state. Since, while thepotential of the scanning line Gi is GH (during the period from time t0to time t5), the switching TFT 615 is in a non-conduction state, acurrent does not flow through the organic EL element 630 and thus theorganic EL element 630 does not emit light.

While the potential of the scanning line Gi is GH, the potential of thedata line Sj is controlled to a data potential Vdata. During thisperiod, the connection point B is connected to the data line Sj throughthe switching TFT 611 and thus the potential at the connection point Bbecomes Vdata. Since, during the period from time t0 to time t1, theswitching TFTs 613 and 614 are in a non-conduction state, when thepotential at the connection point B is changed from Vstd to Vdata, thepotential at the connection point A is also changed by the same amount(Vdata−Vstd).

Then, when at time t1 the potential of the control line Ri is changed toGL, the switching TFT 614 is changed to a conduction state. Thus, theconnection point A is connected to the reference supply wiring line Vsthrough the switching TFT 614 and thus the potential at the connectionpoint A is changed to Vstd. At this time, since the connection point Bis connected to the data line Sj through the switching TFT 611, evenwhen the potential at the connection point A is changed, the potentialat the connection point B remains at Vdata.

The reference potential Vstd of the reference supply wiring line Vs isdetermined such that the driving TFT 610 goes into a conduction statewhen the reference potential Vstd is applied to the gate terminal.Hence, after time t1, the driving TFT 610 is surely in a conductionstate. Note that even when the driving TFT 610 goes into a conductionstate, while the switching TFT 615 is in a non-conduction state, acurrent does not flow through the organic EL element 630 and thus theorganic EL element 630 does not emit light.

Then, when at time t2 the potential of the control line Ri is changed toGH, the switching TFT 614 is changed to a non-conduction state. Thus,the connection point A is disconnected from the reference supply wiringline Vs and thus the potential at the connection point A is fixed. Atthis time, a potential difference (Vstd−Vdata) between the connectionpoints A and B is held in the capacitor 620.

Then, when at time t3 the potential of the control line Wi is changed toGH, the switching TFT 613 is changed to a conduction state. Thus, thegate and drain terminals of the driving TFT 610 are short circuited,whereby the driving TFT 610 forms a diode connection. During the periodfrom time t1 to time t2, the reference potential Vstd is applied to theconnection point A and after time t2 too, the potential at theconnection point A is kept at Vstd by the capacitor 620. Therefore,after time t3 too, the driving TFT 610 is surely in a conduction state.

Also, a current flows out to the common cathode Vcom from the connectionpoint A through the switching TFT 613 and the driving TFT 610 and thusthe potential at the connection point A (the gate terminal potential ofthe driving TFT 610) drops while the driving TFT 610 is in a conductionstate. The driving TFT 610 is changed to a non-conduction state when thegate-source voltage becomes a threshold voltage Vth (positive value)(i.e., the potential at the connection point A becomes (VSS+Vth)).Hence, the potential at the connection point A drops to (VSS+Vth) andthe driving TFT 610 goes into a threshold state.

Then, when at time t4 the potential of the control line Wi is changed toGL, the switching TFT 613 is changed to a non-conduction state. At thistime, a potential difference (VSS+Vth−Vdata) between the connectionpoints A and B is held in the capacitor 620.

Then, when at time t5 the potential of the scanning line Gi is changedto GL, the switching TFTs 612 and 615 are changed to a conduction stateand the switching TFT 611 is changed to a non-conduction state. Thus,the connection point B is disconnected from the data line Sj andconnected to a reference supply wiring line Vs through the switching TFT612. Hence, the potential at the connection point B is changed fromVdata to Vstd and accordingly the potential at the connection point A isalso changed by the same amount (Vstd Vdata; hereinafter, referred to asVC) and becomes (VSS+Vth+VC).

After time t5, the switching TFT 615 is in a conduction state and thusthe current flowing through the common cathode Vcom from the powersupply wiring line Vp through the switching TFT 615 and the driving TFT610 also flows through the organic EL element 630. Although the amountof current flowing through the driving TFT 610 increases or decreasesdepending on the gate terminal potential (VSS+Vth+VC), even when thethreshold voltage Vth is different, if the potential difference VC(=Vstd Vdata) is the same, then the amount of current is the same.Therefore, regardless of the value of the threshold voltage Vth of thedriving TFT 610, a current of an amount according to the potential Vdataapplied to the data line Sj flows through the organic EL element 630 andthus the organic EL element 630 emits light with a specified luminance.

As described above, the pixel circuit 600 includes the driving TFT 610of an n-channel type. Even with the display device according to thepresent embodiment including such a pixel circuit 600, as with the fifthembodiment, the same effect as that obtained in the first embodiment andthe effect of being able to easily perform a peak luminance adjustmentcan be obtained.

SEVENTH EMBODIMENT

FIG. 15 is a circuit diagram of a pixel circuit included in a displaydevice according to the seventh embodiment of the present invention. Apixel circuit 700 shown in FIG. 15 includes a driving TFT 710, switchingTFTs 711 to 715, a capacitor 720, and an organic EL element 730. Theswitching TFTs 711 and 714 are of an n-channel type and other TFTs areof a p channel type.

The pixel circuit 700 is obtained by making a change to the pixelcircuit 500 (FIG. 11) according to the fifth embodiment such that theswitching TFT 514 is connected to a different point. In FIG. 15, aconnection point between the driving TFT 710 and the switching TFTs 713and 715 is referred to as C. In the pixel circuit 700, the switching TFT714 is provided between the connection point C and a reference supplywiring line Vs. Except for the above points, the configuration of thepixel circuit 700 is the same as that of the pixel circuit 500.

FIG. 16 is a timing chart of the pixel circuit 700. FIG. 16 showschanges in potentials applied to a scanning line Gi, control lines Wiand Ri, and a data line Sj and changes in potentials at connectionpoints A and B. In FIG. 16, the period from time t0 to time t4corresponds to one horizontal scanning period. With reference to FIG.16, differences in operation between the pixel circuit 700 and the pixelcircuit 500 will be described below.

The pixel circuit 700 operates in the same manner as the pixel circuit500 (i.e., in the same manner as the pixel circuit 100) during theperiod from time t0 to time t1. When at time t1 the potential of thecontrol line Wi is changed to GL and the potential of the control lineRi is changed to GH, the switching TFTs 713 and 714 are changed to aconduction state. Thus, the connection point A is connected to thereference supply wiring line Vs through the switching TFTs 713 and 714and thus the potential at the connection point A is changed to Vstd.

The reference potential Vstd of the reference supply wiring line Vs isdetermined such that the driving TFT 710 goes into a conduction statewhen the reference potential Vstd is applied to the gate terminal.Hence, after time t1, the driving TFT 710 is surely in a conductionstate. Note that even when the driving TFT 710 goes into a conductionstate, while the switching TFT 715 is in a non-conduction state, acurrent does not flow through the organic EL element 730 and thus theorganic EL element 730 does not emit light.

Meanwhile, when the switching TFT 713 goes into a conduction state, thegate and drain terminals of the driving TFT 710 are short-circuited,whereby the driving TFT 710 forms a diode connection. Hence, a currentflows into the connection point A from the power supply wiring line Vpthrough the driving TFT 710 and the switching TFT 713 and thus thepotential at the connection point A rises by an amount corresponding tothe current flown into. Accordingly, the potential at the connectionpoint A becomes, strictly speaking, a potential (Vstd+β) which is alittle higher than Vstd.

Then, when at time t2 the potential of the control line Ri is changed toGL, the switching TFT 714 is changed to a non-conduction state. Thus,the current flowing into the connection point A from the referencesupply wiring line Vs through the switching TFT 714 is interrupted.Instead of this, a current flows into the connection point A from thepower supply wiring line Vp through the driving TFT 710 and theswitching TFT 713, and the potential at the connection point A (the gateterminal potential of the driving TFT 710) rises while the driving TFT710 is in a conduction state. The driving TFT 710 is changed to anon-conduction state when the gate-source voltage becomes a thresholdvoltage Vth (negative value) (i.e., the potential at the connectionpoint A becomes (VDD+Vth)). Thus, the potential at the connection pointA rises to (VDD+Vth) and the driving TFT 710 goes into a thresholdstate.

After time t3, the pixel circuit 700 operates in the same manner as thepixel circuit 500 does after time t4. After time t4, regardless of thevalue of the threshold voltage Vth of the driving TFT 710, a current ofan amount according to the data potential Vdata flows through theorganic EL element 730 and thus the organic EL element 730 emits lightwith a specified luminance.

As described above, in the pixel circuit 700, the switching TFT 714 isconnected to the reference supply wiring line Vs and the drain terminalof the driving TFT 710 (a current input/output terminal connected to theswitching TFT 713). Even with the display device according to thepresent embodiment including such a pixel circuit 700, as with the fifthembodiment, the same effect as that obtained in the first embodiment andthe effect of being able to easily perform a peak luminance adjustmentcan be obtained.

Generally, in a pixel circuit, since a leakage current flows through aswitching element, a charge held in a capacitor increases or decreaseswhile an electro-optical element emits light and thus there is a problemthat the luminance of the electro-optical element varies with elapsedtime. Here, the number of switching TFTs connected to the connectionpoint A is one in the pixel circuit 700 while two in the pixel circuit500. As such, since in the pixel circuit 700 the number of switchingTFTs connected to the gate terminal of the driving TFT 710 is smaller, aleakage current is also little and a charge held in the capacitor 720 isalso less likely to fluctuate. Hence, according to the display deviceaccording to the present embodiment, fluctuations in the gate terminalpotential of the driving TFT 710 can be suppressed and display qualitycan be enhanced.

Note that although the pixel circuit 700 is obtained by making a changeto the pixel circuit 500 according to the fifth embodiment such that oneterminal of a switching TFT is connected to the reference supply wiringline Vs, and the other terminal is connected to a drain terminal of adriving TFT, the same change may be made to the pixel circuits accordingto the first to fourth and sixth embodiments. Even with a display deviceincluding a pixel circuit to which a change has been made, as with theseventh embodiment, fluctuations in the gate terminal potential of adriving TFT can be suppressed and display quality can be enhanced.

As described above, according to the display devices according to theembodiments, variations in the threshold voltage of a driving TFT can beproperly compensated for, unwanted light emission from an organic ELelement can be prevented, the contrast of a display screen can beenhanced, and the lifetime of the organic EL element can be extended.The present invention is not limited to the embodiments and the featuresof the embodiments can be appropriately combined.

INDUSTRIAL APPLICABILITY

A display device of the present invention obtains the effect of beingable to properly compensate for variations in the threshold voltage of adrive element and to prevent unwanted light emission from anelectro-optical element, and thus, can be used in various displaydevices including electric current driving type display elements, suchas an organic EL display and an FED.

1. An electric current driving type display device comprising: aplurality of pixel circuits arranged so as to correspond to respectiveintersections of a plurality of scanning lines and a plurality of datalines; a scanning signal output circuit that selects a write-targetpixel circuit using the scanning line; and a display signal outputcircuit that provides potentials according to display data to the datalines, wherein each of the pixel circuits includes: an electro-opticalelement provided between a first power supply wiring line and a secondpower supply wiring line; a drive element provided in series with theelectro-optical element and between the first power supply wiring lineand the second power supply wiring line; a capacitor having a firstelectrode connected to a control terminal of the drive element; a firstswitching element provided between a second electrode of the capacitorand the data line; a second switching element provided between thesecond electrode of the capacitor and a predetermined power supplywiring line; a third switching element provided between the controlterminal and one current input/output terminal of the drive element; anda fourth switching element having one terminal connected to a thirdpower supply wiring line and having an other terminal connected directlyor through the third switching element to the control terminal of thedrive element.
 2. The display device according to claim 1, wherein apotential that brings the drive element into a conduction state isapplied to the third power supply wiring line.
 3. The display deviceaccording to claim 1, wherein the fourth switching element is providedbetween the third power supply wiring line and the control terminal ofthe drive element.
 4. The display device according to claim 3, whereinwhen writing to the pixel circuit, during a first period, the first andfourth switching elements are controlled to a conduction state and thesecond and third switching elements are controlled to a non conductionstate, then, during a second period, the fourth switching element iscontrolled to a non-conduction state and the third switching element iscontrolled to a conduction state, and then, during a third period, thefirst and third switching elements are controlled to a non-conductionstate and the second switching element is controlled to a conductionstate.
 5. The display device according to claim 1, wherein the fourthswitching element is provided between the third power supply wiring lineand the current input/output terminal of the drive element, the terminalbeing connected to the third switching element.
 6. The display deviceaccording to claim 5, wherein when writing to the pixel circuit, duringa first period, the first, third, and fourth switching elements arecontrolled to a conduction state and the second switching element iscontrolled to a non-conduction state, then, during a second period, thefourth switching element is controlled to a non-conduction state, andthen, during a third period, the first and third switching elements arecontrolled to a non-conduction state and the second switching element iscontrolled to a conduction state.
 7. The display device according toclaim 1, wherein the second switching element is provided between thefirst power supply wiring line and the second electrode of thecapacitor.
 8. The display device according to claim 7, wherein a controlterminal of the fourth switching element is connected to the third powersupply wiring line, and a potential of the third power supply wiringline is switched between a potential that brings the drive element intoa conduction state and a potential that brings the fourth switchingelement into a non-conduction state.
 9. The display device according toclaim 1, wherein the second switching element is provided between thethird power supply wiring line and the second electrode of thecapacitor.
 10. The display device according to claim 9, wherein apotential of the third power supply wiring line is configured to becontrollable.
 11. The display device according to claim 1, wherein eachof the pixel circuits further includes a fifth switching elementprovided between the drive element and the electro-optical element. 12.The display device according to claim if wherein when writing to thepixel circuit, a potential of the second power supply wiring line iscontrolled such that an applied voltage to the electro-optical elementis lower than a light-emission threshold voltage.
 13. The display deviceaccording to claim 1, wherein the electro-optical element includes anorganic EL element.
 14. The display device according to claim 1, whereinthe drive element and all of the switching elements in the pixel circuitinclude thin-film transistors.
 15. The display device according to claim14, wherein the drive element and all of the switching elements in thepixel circuit include thin-film transistors of a same channel type.